Surface drive heads for progressing cavity pumps require a stuffing box to seal crude oil from leaking onto the ground where the polished rod passes from the crude oil passage in the wellhead to the drive head.
Due the abrasive sand particles present in crude oil and poor alignment between the wellhead and stuffing box, leakage of crude oil from the stuffing box is common in some applications. This costs oil companies money in service time, down time and environmental clean up. It is especially a problem with heavy crude oil wells in which the oil is often produced from semi-consolidated sand formations since loose sand is readily transported to the stuffing box by the viscosity of the crude oil. It is very difficult to make stuffing boxes that last as long as desirable by oil production companies. Costs associated with stuffing box failures are one of the highest maintenance costs on many wells.
Conventional stuffing boxes are mounted below the drive head. Conventional stuffing boxes are typically separate from the drive head and are mounted in a wellhead frame such that they can be serviced from below the drive head without removing it. A conventional stuffing box uses braided packing that is split so it can be replaced while the polished rod stays inside the stuffing box. Since conventional stuffing boxes seal against the polished rod, which is subject to wear, and due to poor alignment of the polished rod to the stuffing box, leakage becomes somewhat inevitable. Due to this experience, users tend to expect stuffing box leakage if the stuffing box uses braided packings.
In order to reduce or eliminate the leakage, high-pressure lip seals have been used running against a hardened sleeve rather than against the polished rod. Grenke in Canadian Patent No. 2,095,937 issued Dec. 22, 1998 shows a typical stuffing box employing lip seals. These stuffing boxes are known in the industry as environmental stuffing boxes because they do not leak at all until the lip seals fail. Since these high-pressure lip seals are not split and are mounted below the drive head, they cannot be replaced with the polished rod in place so the drive head must be removed to service the stuffing box. Since the drive head must be removed to service the lip seals, the wellhead frame has been eliminated and the stuffing box is bolted directly to the bottom of the drive head on many drive heads now being produced. This type of stuffing box directly mounted to the drive head is shown in the above referenced Grenke patent. This product is made by Grenco Industries. These types of stuffing boxes are referred to as integral.
There are many types of rotary lip seals that might be applied to stuffing boxes for progressing cavity pumped wells. Grenco and other competitors have had some field success with the type described as flanged variseals in the American Variseal catalog. American Variseal is a member of Busak and Shamban Inc. This type of seal is made by a number of competitors. Generally these seals are machined from reinforced Teflon and they have a preload spring between two lips. The flange is convenient for mounting the seal and stabilizing it. Since the seals are Teflon based, they can operate without lubrication.
Servicing of stuffing boxes is time consuming and difficult. In order to service the environmental or integral stuffing boxes, the drive head must be removed which necessitates using a rig with two winch lines, one to support the drive head and the other to hold the polished rod. To save on rig time, the stuffing box is typically replaced and the original stuffing box is sent back to a service shop for repair.
Recently, Oil Lift Technology Inc. has introduced top mounted stuffing boxes to the industry, which allow the stuffing box to be serviced from on top of the drive head without removing the drive head from the well. These types of stuffing box are shown in Hult Canadian patent application 2,350,047 (the “Oil Lift Stuffing Box”). These top mounted stuffing boxes use a flexibly mounted “floating” standpipe around which is a bearing supported shaft carrying the rotary stuffing box seals. Typically the primary rotary stuffing box seal is braided packing since it has proven to last for a long time when running against the hardened, flexibly mounted standpipe. Braided packings made from Teflon and graphite fibres and been used most frequently. Kevlar cornered packings are often used for the first and last packing rings to prevent extrusion. Packings of this type are generally self lubricating which can also be an advantage in the present invention. Because the standpipe floats, it self aligns to the packing, reducing or eliminating run out and leakage compared to conventional stuffing boxes. Packings have very low resilience so reduction of run out is very important in prevention of leakage. In some cases the stuffing box is counter-pressurized, preferably by lubricating oil at a higher pressure than the wellhead pressure so if there is any leakage through the primary rotary stuffing box seal, lubricating oil goes down the well rather than allowing well fluids to leak into the drive head. In the most difficult applications, the use of pressurized lubricating oil has proven very beneficial in extending stuffing box seal life, demonstrating many times the stuffing box seal life compared to non-pressurized stuffing boxes.